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LC Filter Calculator

Electronics & PhysicsCircuits and Components
Filter type
Cutoff frequency (Hz) —
Cutoff in kHz
—
Angular frequency (rad/s)
—
Characteristic impedance (Ω)
—
Passband
—
Attenuation one decade into the stopband
—

An LC filter's cutoff frequency is fc = 1 ÷ (2π√(LC)). With a 10 mH inductor and a 1 µF capacitor, √(LC) is 0.0001 s, so fc is about 1,591.55 Hz. The same pair has a characteristic impedance √(L/C) of 100 Ω, the load resistance that keeps the response flat up to the corner.

About this tool

Designers of speaker crossovers, power-supply output stages and RF front ends use a two-element LC section when they need a 12 dB-per-octave slope without an op-amp. Enter the inductor and capacitor with their own units and the tool returns the corner frequency in hertz and kilohertz, the angular frequency, and the characteristic impedance the section was designed around. The filter type switch only changes which side of the corner passes; the corner itself is identical for low-pass and high-pass layouts. One limit: the figures assume ideal parts and a resistive load equal to √(L/C). Inductor winding resistance, capacitor ESR and a mismatched load shift the peak and the Q, so a real circuit can show a bump or a soft knee near the corner.

How to use it

  1. Choose low-pass or high-pass

    Pick the layout: series inductor with shunt capacitor passes low frequencies, series capacitor with shunt inductor passes high ones.

  2. Enter the inductance

    Type the value and choose nH, µH, mH or H in the unit menu inside the field.

  3. Enter the capacitance

    Type the value and pick pF, nF, µF or F from the attached menu.

  4. Read the corner and impedance

    The cutoff, angular frequency and √(L/C) update as you type, with the working shown underneath.

Examples

10 mH with 1 µF, low-pass

Filter type
Low-pass
Inductance (L)
10
Inductance unit
mH
Capacitance (C)
1
Capacitance unit
µF

Result Cutoff frequency (Hz): 1,591.55
Cutoff in kHz: 1.5915
Angular frequency (rad/s): 10,000
Characteristic impedance (Ω): 100
Passband: Passes below 1,591.55 Hz
Attenuation one decade into the stopband: −40 dB (ideal, matched load)

  1. L = 1.0000e-2 H, C = 1.0000e-6 F
  2. √(L × C) = 1.0000e-4 s
  3. fc = 1 ÷ (2π × 1.0000e-4) = 1,591.55 Hz
  4. Z0 = √(L ÷ C) = 100 Ω — terminate the filter near this resistance for a flat response

A common audio-band pair: the cutoff lands near 1.6 kHz and the filter wants a 100 Ω load.

100 µH with 100 nF, high-pass

Filter type
High-pass
Inductance (L)
100
Inductance unit
µH
Capacitance (C)
100
Capacitance unit
nF

Result Cutoff frequency (Hz): 50,329.21
Cutoff in kHz: 50.3292
Angular frequency (rad/s): 316,227.77
Characteristic impedance (Ω): 31.62
Passband: Passes above 50,329.21 Hz
Attenuation one decade into the stopband: −40 dB (ideal, matched load)

  1. L = 1.0000e-4 H, C = 1.0000e-7 F
  2. √(L × C) = 3.1623e-6 s
  3. fc = 1 ÷ (2π × 3.1623e-6) = 50,329.21 Hz
  4. Z0 = √(L ÷ C) = 31.62 Ω — terminate the filter near this resistance for a flat response

Typical switching-supply parts: the corner sits at about 50.3 kHz with an impedance near 31.6 Ω.

How it is calculated

fc = 1 ÷ (2π√(L·C)); Z0 = √(L ÷ C); ω = 2π·fc

fc
cutoff (corner) frequency in hertz
L
inductance in henries
C
capacitance in farads
Z0
characteristic impedance in ohms
ω
angular frequency in radians per second

Both parts are first converted to base SI units. At fc the inductive reactance 2πfL equals the capacitive reactance 1/(2πfC); solving that equality gives the cutoff formula. Dividing the two reactances instead gives Z0, which is independent of frequency. When source and load both equal Z0, a single LC section behaves like a second-order Butterworth-style filter whose output falls about 40 dB for each decade past the corner.

When not to use it

  • Do not use it for a filter with three or more reactive elements; those need ladder tables.
  • It does not model inductor resistance or capacitor ESR, which matter in power filters.
  • For a parallel tank driven as a resonator, bandwidth and Q need the circuit resistance too.

Common mistakes

  • Entering microfarads while the unit menu still says nanofarads moves the corner by a factor of about 31.6.
  • Leaving the filter unloaded produces a sharp resonant peak at the corner instead of a flat knee.
  • Picking L and C for the right corner but ignoring Z0 leaves the section mismatched to the speaker or load.

Frequently asked questions

Why is the cutoff the same for low-pass and high-pass?

The corner is where the two reactances are equal, and that depends only on the product L·C. Exchanging the positions of the inductor and capacitor changes which band passes, not where the crossover between them sits.

What does the characteristic impedance tell me?

It is √(L/C), the resistance the filter expects to see at its output. A 100 Ω design driving a 100 Ω load gives a smooth knee; driving 1 kΩ instead raises Q and produces a peak near the corner that can exceed several decibels.

How steep is an LC filter's roll-off?

A single inductor plus capacitor is second order, so the response falls at roughly 12 dB per octave, or 40 dB per decade, once you are well past the corner. An RC section falls only 6 dB per octave.

How do I pick L and C for a target frequency and load?

Combine the two formulas: L = Z0 ÷ (2π·fc) and C = 1 ÷ (2π·fc·Z0). For a 1 kHz corner into 8 Ω, that gives about 1.27 mH and 19.9 µF, which you would then round to stocked values and check here.

Is this the same as the LC resonant frequency?

The number is the same, because both come from 1 ÷ (2π√(LC)). The resonance view focuses on a tank circuit oscillating, while the filter view adds the impedance the section should be terminated with and which band it passes.

Why does my measured corner differ from the result?

Component tolerances of 5 to 20 percent, inductor winding resistance and core saturation at high current all move the real response. A 10 percent error in both parts shifts the corner by about 10 percent.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 18). LC Filter Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/circuits-and-components/lc-filter-calculator/

MLA

Tools Directory Editorial Team. “LC Filter Calculator.” Tools Directory, 18 Oct. 2026, https://tools.milunir.com/electronics-physics/circuits-and-components/lc-filter-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “LC Filter Calculator.” Tools Directory. Last modified October 18, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/circuits-and-components/lc-filter-calculator/.

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